skygrad

Realistic sky gradient PNGs from latitude, longitude, and time. Zenith at the top, horizon at the bottom — one color per row, or the sun's glow lobe across the frame when you pass facing. The sun's position drives every color in the image; the sun itself is never drawn — no disc, no moon, no stars, no clouds. Zero runtime dependencies.

Usage

from datetime import datetime
import skygrad

# One-shot PNG bytes
png = skygrad.render(lat=34.05, lon=-118.24,
                     when=datetime(1994, 6, 21, 19, 30),
                     width=512, height=1024)

# Or write straight to a file
skygrad.write_png("dusk.png", lat=34.05, lon=-118.24,
                  when=datetime(1994, 6, 21, 19, 30))

# Layered access: colors without pixels
sky = skygrad.Sky.at(lat=34.05, lon=-118.24, when=datetime(1994, 6, 21, 19, 30))
sky.solar_elevation          # degrees; e.g. streetlights on below -4
sky.color(0.0)               # (r, g, b) at the zenith; 1.0 is the horizon
sky.stops()                  # [(u, (r, g, b)), ...] — build a CSS gradient

Facing (azimuthal glow)

Pass a compass direction and the image gains the sun's glow lobe — brightest toward the sun, fading with angular distance, gone when you face away. Without facing, output is byte-identical to skygrad 0.1.

sky = skygrad.Sky.at(lat=34.05, lon=-118.24, when=datetime(1994, 6, 21, 19, 40))
sky.solar_azimuth                      # ≈ 303° at this dusk — west-northwest
toward = sky.png(facing=sky.solar_azimuth)            # sunset glow, centered
away = sky.png(facing=(sky.solar_azimuth + 180) % 360)  # plain dusk gradient
wide = sky.png(facing=0.0, fov=360.0, width=2048)     # full panorama strip

facing is degrees clockwise from true north (any finite value, normalized mod 360; radians callers use math.degrees()). fov is the horizontal span in degrees, default 90, valid (0, 360] — and requires facing.

A facing render is computed per pixel, so it's capped at ~4 million pixels (width × height ≤ 2048×2048); go larger and it raises ValueError. Tile, shrink, or drop facing (the plain vertical gradient is per-row and has no such cap). Requires Python 3.11+.

Time semantics (read this)

when is a full datetime — the date matters (June and December at the same hour give completely different skies).

• Timezone-aware datetimes are honored exactly.
• Naive datetimes mean local mean solar time at the given longitude: sundial time, where 12:00 puts the sun on the meridian. This is deliberate — it makes "local" meaningful for fictional places — but it can differ from civil wall-clock time by an hour or more. If you want wall-clock local time, attach a tzinfo.

Determinism

Same inputs → same decoded pixels for a given skygrad.MODEL_VERSION (and byte-identical PNGs within one zlib build). MODEL_VERSION bumps whenever any input would render different colors, so golden tests in consumers break deliberately, never silently. Compare decoded pixels, not compressed bytes.

Development

uv sync
uv run ruff check . && uv run ruff format --check .
uv run mypy
uv run pytest -q

Golden skies are pinned under tests/golden/; regenerate deliberately with uv run pytest --regen-golden and bump MODEL_VERSION if colors changed. Regeneration rewrites all ten goldens, but on one zlib build the untouched ones are byte-identical rewrites — so a deliberate single-color change surfaces as exactly the goldens it should. The module map and invariants live in ARCHITECTURE.md, and the design rationale (theory of operation) in THEORY.md.